The present disclosure relates generally to integrated circuit devices, and more specifically, to a self-aligned bipolar transistor.
A bipolar junction transistor (BJT or bipolar transistor) is a type of transistor that relies on the contact of two types of semiconductor for its operation. One type of semiconductor is formed with positive-type dopants, and is therefore referred to as a P-type semiconductor, while another type of semiconductor is formed with negative-type dopants, and is therefore referred to as an N-type semiconductor. A bipolar junction transistor usually includes two back-to-back p-n junctions that share a thin common region. In other words, a bipolar junction transistor typically includes three regions, two outer regions commonly known as “emitter” and “collector” respectively, and a middle region commonly known as “base”. Electrical connections are generally made to all three regions.
Bipolar junction transistors can be used as amplifiers, switches, or in oscillators. Bipolar junction transistors come in two types, or polarities, known as PNP and NPN, with the P and the N representing the different doping types of the three main terminal regions (collector, base, and emitter). An NPN transistor comprises two semiconductor junctions that share a thin P-doped anode region, and a PNP transistor comprises two semiconductor junctions that share a thin N-doped cathode region.
Improvement in transistor performance, especially its operation speed, is generally considered as essential for achieving improved performance of an integrated circuit wherein various types of transistors are normally used. Bipolar transistors with a silicon germanium (“SiGe”) intrinsic base may be capable of delivering performance required for such integrated circuit. To achieve higher electrical performance, these transistors typically have a doped polysilicon extrinsic base layer on top of or adjacent to the epitaxially grown intrinsic SiGe base. So far, SiGe-HBTs have demonstrated cut-off frequency (fT) of up to 400 GHz. The emitter to collector transit time of such a transistor is typically reduced by optimizing the Ge/Si ratio, doping profile, and film thickness of the epitaxially grown intrinsic SiGe base. Such devices are used, for example, in integrated circuits fabricated for high performance mixed signal applications.